The present invention relates, in general, to a method and arrangement for measuring a reflective, two-dimensional structure in a layer of a volume by means of evaluating sonic or ultrasonic echoes, and, in particular to a new and useful method and arrangement for measuring a velocity field of such a structure.
In particular in connection with the ultrasonic pulse Doppler technique for measuring fluid velocities in fluid carrying scattering centers, it has been known for more than two decades (Baker, "Pulsed Ultrasonic Blood Flow Sensing", IEEE Trans. SU-17/3, p. 170 (1970)) to determine the velocity of the fluid by evaluating the Doppler frequency of an echo signal originating from a given distance or its phasing with respect to a stable reference signal. Thereby, due to the Doppler effect only the velocity component pointing in the beam-return direction is determined.
Due to the different distances existing at a two-dimensional layer with respect to a receiver, a two-dimensional structure could not be measured in accordance with the above cited technique even when the known method was applied to a stationary structure.
In a further developed called a multi-channel apparatus (McLeod, "A Multiple Gate Pulsed Doppler Flowmeter", Proc. IEEE Ultrasonic Symp. Miami, (1970)), echoes exclusively from different distances were sequentially evaluated and the results were displayed quasi-simultaneously according to a velocity profile along the beam of acoustic irradiation. With this method too, and due to the above cited distance conditions at a two-dimensional layer and to the actually one-dimensional measuring principle, measuring of the type stated above is not possible.
From DE-PS 24 06 630 a method for measuring a velocity profile of a flowing fluid carrying reflective structures based on the evaluation of sonic or ultrasonic echoes became known, by which determination of the velocity profile became possible without actually preselecting the distances. With this method too, it is not possible to measure a two-dimensionally extended layer with respect to reflective structures due to the above stated distance conditions, in the event the approach is applied to stationary structures.
Furthermore it is known to deflect a sonic echo beam differently, by means of so-called phased array sector scanners. The sonic echo beam is deflected in a plane containing the transmitter/target axis by different transmitter stimulation to determine scatter center velocities along the currently selected axis of acoustic irradiation. This is done sequentially and in different directions in the plane. In this way, the two-dimensional velocity field is sampled in this plane and in the "B mode" known to the person skilled in this art.
With this method, so called echography is only possible in a plane which is in the plane with the target axis, even for stationary structure. When using this method for its primary purpose, namely for the measurement of a velocity profile, a further problem is encountered: For a real-time measurement, the measuring time for sampling in the plane must be limited so that either only short measuring time periods are possible in each current beam direction or, when lengthening these time periods, a measurement is only possible in a few sampling directions.
In the case of a so-called Color Doppler Apparatus (Chihiro, "Real-Time Two-Dimensional Blood Flow Imaging Using an Autocorrelation Technique", IEEE Trans. on Sonics and Ultrasonics, Vol. SU 32, No. 3, (1985)), primarily the measuring time in the respective beam directions is strongly limited so that, due to the lack of definition, the Doppler frequency can only be determined very imprecisely.